1. Field of the Invention
This invention relates to an aqueous binder composition based on polyisocyanates and water soluble or water dispersible polyester resins containing urethane, carboxylate and hydroxyl groups and polyisocyanates and to a process for the preparation of such binder compositions.
2. Description of the Prior Art
Two-component polyurethane coating compositions are known and are very important for use as coatings due to their excellent properties but they have the disadvantage that their processing requires substantial quantities of organic solvents which must subsequently be removed, e.g., by burning. In many fields of application, especially those which are non-industrial, the disposal of the organic solvents cannot be carried out and the demand is therefore even greater for coating compositions which have a high solids content and above all are dilutable with water in order to reduce the release of solvents and, thus, environmental pollution as much as possible.
The use of water as the liquid phase in two-component polyurethane lacquers having free isocyanate groups is normally not possible since isocyanate groups react not only with alcoholic hydroxyl groups but also with water to form urea and carbon dioxide. The pot life, the processing time and the quality of the resulting coatings are thereby generally reduced to a level which is not acceptable in practice.
One possible solution is described in DE-OS 3,829,587, in which the selected polyhydroxypolyacrylates are combined with polyisocyanates containing free isocyanate groups to form aqueous two-component systems.
It has now surprisingly found that special aqueous polyester resins containing urethane, carboxylate and hydroxyl groups may also be used together with polyisocyanates containing free isocyanate groups in aqueous two-component binders.